[47.04] The Kinematics of the Damped Lyman Alpha Protogalaxies

J. X. Prochaska, A. M. Wolfe (UCSD)

We present the first results of an ongoing program
to investigate the kinematic characteristics of high redshift
damped Ly\alpha systems. Because damped Ly\alpha
systems are widely believed
to be the progenitors of current massive galaxies,
an analysis of their kinematic history allows
a direct test of galaxy formation scenarios.
In turn, we aim to constrain different cosmogonies by making
comparisons with both Press-Schechter predictions and N-Body
simulations.
We have collected a kinematically unbiased sample of 17 high
signal-to-noise ratio, high resolution damped Ly\alpha spectra
taken with HIRES on the 10 m W.M. Keck Telescope. Our
study focuses on the unsaturated, low-ion transitions (e.g.
Si II 1808) of these systems which reveal
their kinematic traits. The profiles
exhibit a nearly uniform distribution of velocity widths
ranging from \approx 20 - 200 km/s and a relatively
high degree of asymmetry. In an attempt to explain these
characteristics, we introduce several physical models,
which have previously been attributed to damped Ly\alpha systems,
including rapidly rotating thick disks, slowly rotating hot disks,
massive isothermal halos, and a hydrodynamic spherical accretion
model.
Using standard Monte Carlo techniques, we run sightlines through
these model systems to derive simulated low-ion profiles.
We develop 4 statistical tests to distinguish between the models
which focus on the symmetry and velocity widths of the profiles.
Comparing the distributions of values from the simulated profiles
with those calculated from the observed profiles, we determine that the
rapidly rotating thick disk model is the only tested
model consistent with the data at high confidence levels.
A Relative Likelihood Test
of the rapidly rotating thick disk model indicates the disks must have
large rotation speeds (v_rot > 180 km/s at the 99% c.l.)
and be relatively thick (the ratio of the vertical scale height to
the radial scale length, h/R_d > 0.1). These characteristics
are highly suggestive of the thick disk of the Milky Way.

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